Small accounting firms, law offices and agencies get calls at the worst time: after the office closes. A client opens an IRS letter at 7 PM. A business owner wants a new accountant and calls the first firm that picks up. If your line goes to voicemail, many of those callers never leave a message. They call the next name on the list.
This is a walkthrough of an after-hours AI receptionist built in ElevenLabs Agents: one that answers, knows your hours, and takes a clean message with an urgent flag for the morning. I will show the business math first, then the prompt and the build.
What a missed call costs
Take a firm that misses two new-client calls a week after hours. If each new client is worth a few hundred dollars a year, that is tens of thousands of dollars of work a year going to whoever answered. Your numbers will differ, so work them out: calls missed after hours per week, times the share that were new clients, times what a client is worth to you in a year.
Existing clients are the other half. A worried client with a deadline who gets voicemail wonders whether anyone is watching. And on Monday morning someone on your team has to play a pile of voicemails, many with no number and no reason for the call.
An AI receptionist does not replace your team. It covers the hours when nobody is there, answers the routine questions and hands you a clean message. It should never give advice, and it should always tell callers it is an AI.
The example agent
The example is a made-up firm, Hartwell Accounting: tax returns, bookkeeping and payroll for local businesses and families, open Monday to Friday, 9 AM to 5 PM. The receptionist is called Casey.
In the test call, a caller named Dana asks when the office opens on Monday and whether the firm does payroll. Casey answers from a short document. Then Dana says she got a letter from the IRS and is worried. Casey does not give tax advice. It moves into the message steps, takes her name and number, hears the notice has a Friday deadline, and marks the message urgent for a callback first thing the next business morning.
The build, in five parts
1. Create the agent. In ElevenLabs, open Agents, click the + next to Agents, choose Blank Agent, name it and create it.
2. Write the job. On the Agent page, replace the default system prompt. This is the core, so here is a trimmed version of the shape I use:
You are Casey, the after-hours receptionist for Hartwell Accounting.
The office is closed. You are an AI assistant, and you say so.
You can answer questions about hours, services and what to bring, using
only the knowledge base. If it is not in the knowledge base, say you will
pass the question to the team.
You never give tax or legal advice, never quote prices, never ask for a
Social Security number or bank account number.
To take a message, follow these steps in order:
1. Ask for the caller's name.
2. Ask for the best phone number and read it back.
3. Ask what the call is about in one sentence.
4. Ask whether it is urgent. Urgent means a notice or deadline this week.
5. Say someone will call back the next business morning.
6. Ask if there is anything else.
The numbered order is what makes every message have the same fields. And the concrete example of "urgent" matters: a notice with a deadline this week is far more reliable than the word "urgent" alone.
3. Write the first message. Name the firm, say the office is closed, say Casey is an AI assistant, and say the call may be recorded.
4. Teach it your office. Under Configure, open Knowledge Base, click Add document, choose Create Text, name it and paste your hours, services, new-client process and a short FAQ. Keep it short and approved by you. The agent is only as good as this document.
5. Publish and test. Click Publish, review and publish. Open Preview and press the phone button, or use the chat box on the same page if you cannot use a microphone. Ask a quick question, ask for a callback about an IRS letter, then give a name, number and deadline. Open Conversations under Monitor and read the summary, then the full transcript.
Rules that keep it safe
- Tell callers in the first message that they are talking to AI and that the call may be recorded. ElevenLabs asks for AI disclosure, and recording consent rules vary by state, so check yours. This is not legal advice.
- Keep it from giving tax or legal advice, quoting prices that are not in the document, or promising callback times you cannot keep.
- Never let it collect a Social Security number or a bank account number by phone.
- Test the questions you most want it to refuse before real callers reach it.
What it costs
ElevenAgents is billed by call minutes. On ElevenLabs' pricing page at the time of writing, the Free plan includes 15 minutes of calls a month, Starter is $6 a month with 75 minutes, and Creator is $22 a month with 275 minutes. Extra minutes are $0.08 each. The language model is charged on top, and a phone provider such as Twilio bills separately once you connect a real number. Testing in the browser needs no phone number. Prices change, so check the pricing page first.
Before and after
Before: voicemails with no number, a Monday-morning hunt, and new clients who called someone else. After: every after-hours call answered, routine questions handled, and a short summary per call with the urgent ones marked. The message still waits in Conversations until someone opens it, so the next improvement is sending each message to your team the moment the call ends.
Build it step by step
The full lesson has 26 steps and a practice copy of the dashboard, so no account is needed and no real call is made: Never Miss an After-Hours Client Call: An AI Receptionist for a Small Office (ElevenLabs).
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